BBcode [right] align

[someonecantcode]

supersedes #12431 resolves #12356

in #12431, kama talks about rewriting the <center> implementation since it is deprecated but i don't know if i should do something about it. Quick question, if this gets merged, should i update the documentation in https://github.com/ppy/osu-wiki/blob/master/wiki/BBCode/en.md ?

Tested for main audio, spoilerboxes, videos results:

[peppy]

Also seen in https://github.com/ppy/osu-web/pull/12431.